Flame propagation speeds of conventional gasolines suitable for use in spark ignition engines have been measured by various means under various conditions. When a fuel/air ratio in a spark ignition engine is close to the stoichiometric ratio, it is necessary to maintain maximum pressure at the time of combustion at a level lower than the intrinsic maximum pressure to avoid surface ignition, self ignition or the like. Because of this, the time of ignition is spark-advanced from top dead center. In this instance, the term "spark advance" is used to express a crank angle at the time of ignition in advance of the compression top dead center, whose crank angle is defined as 0.degree.. For example, an expression "10.degree. spark advance of ignition" means ignition at 10.degree. crank angle in advance of the compression top dead center. Such an ignition spark advance, however, causes an increase in the combustion pressure during the compression stroke, which results in power loss and reduction of thermal efficiency. In addition, when the fuel/air ratio is too small or too large, the flame propagation speed becomes low, the power decreases sharply and the ignitability becomes poor, thus causing an increase in cycle fluctuation (which means burning fluctuation of each cycle, cyclic variation in combustion duration, maximum pressure or etc. which evaluated as standard deviations). As a consequence, the flame propagation speed and ignitability of conventional gasoline cannot solve such problems of power loss and cycle fluctuation.
In the theoretical cycle of a spark ignition engine (Otto cycle), it is considered in general that the maximum power is obtained when the flame propagation speed of the fuel/air mixture reaches infinity, and the ignition is effected at the top dead center of the compression stroke, followed by instant completion of combustion. Accordingly, it is desirable to use a gasoline fuel which has a higher flame propagation speed than that of conventional gasoline, so that the spark advance can be reduced and ignition can be effected at a crank angle close to the top dead center.
Burning velocity and inflammability limit are physicochemical constant of each compound. These values at atmospheric temperature and pressure have been measured in accordance with the NACA (National Advisory Committee for Aeronautics) method, and the like, revealing the existence of oxygen-containing organic compounds which have high burning velocity and broad inflammability ranges. These data, however, have been obtained from a safety engineering point of view, with no discussion about these oxygen-containing organic compounds with regard to their flame propagation speeds, ignitabilities and the like in a spark ignition engine.
Recently, a constant-volume combustion apparatus has been developed for use in the evaluation of combustion properties of liquid fuel oil (Japanese Patent Application No. 3-1550954, which is hereby incorporated by reference), together with experimental techniques for simple comparative measurement of flame propagation speed and ignitability of liquid fuel at desired fuel/air ratio under certain conditions.
This combustion apparatus comprises a combustion chamber as the main body, equipped with two observation windows on opposite sides. The inside of the main body includes a closed combustion chamber, a heater attached to the outer wall of the combustion vessel, a thermocouple for use in the detection of temperature in the combustion chamber, a liquid fuel oil feeder as a means to supply the combustion chamber with a desired volume of liquid fuel oil, an air supply means to supply the combustion chamber with air, an agitator achieving homogeneous mixtures movable in the combustion chamber, and a spark plug which can discharge a spark in the combustion chamber. Using this combustion apparatus, flame propagation speed can be measured through the observation window, making use of a laser beam refraction method or the like, and combustion characteristics of liquid fuel oil can be evaluated at a laboratory level.
The laser beam refraction method means as follows. A Herium-Neon laser light was split into three beams which passed through the combustion chamber and were detected by high-sensitivity photodiodes. As a flame front which had a high density gradient arrived at an individual beam, the bean was deflected from its course by refraction. Then the laser light reaching each photodiode decreased. The signals from all of the photodiodes were monitored by a digital oscilloscope. The period from ignition to the time of the flame front arriving at the each beam was measured.
FIG. 4 is a whole view of the constant-volume combustion apparatus and FIG. 5 is a partial enlarged view of the combustion vessel.
It is known that, when a fuel-air mixture consisting of air and a multi-component fuel such as gasoline is subjected to combustion in a combustion chamber, variation in the formation of the fuel-air mixture and differences in the ignitability in each cycle become important factors with regard to the aforementioned cycle fluctuation in a spark ignition engine. As a consequence, it would be advantageous if certain fuel additives and fuel blends were available which minimized fluctuation of combustion conditions in each cycle and stabilized combustion. These additives must be effective even under conditions when variation in the formation of the fuel-air mixture and differences in ignitability occur, such as when the fuel/air ratio is too small or too large, or during constant speed driving.
The ignitability is evaluated by the period of ignition lag or the formation of a misfire, which is measured, for example, by the time of from ignition to 10% mass burning rate, and when a misfire is occurred, this time is zero.
With regard to additives useful for the improvement of ignitability of a lean fuel-air mixture, JP-A-62-1785 corresponding to U.S. Pat. No. 4,765,800 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) discloses that ignitability can be improved by the use of, for example, alkali metal salts or alkaline earth metal salts of succinic acid derivatives, which improve ignition lag by shortening flame traveling time from the spark plug gap to the 10 mm distant laser beam without contaminating the inside of the engine. However, metal moieties contained in these compounds are discharged together with exhaust gas, and the discharged metal moieties not only accumulate in the exhaust system but also are discharged further into the air, thus requiring an environmental countermeasure. Also, it is known that these discharged metal moieties degrade the activity of catalysts which are present in the exhaust gas treatment system. In addition, only ignitability is evaluated in the cited '785 patent application, with no disussion of flame propagation speed.